Stock material or miscellaneous articles – Longitudinally sectional layer of three or more sections – Next to unitary sheet of equal or greater extent
Reexamination Certificate
2000-12-27
2004-05-25
Morris, Terrel (Department: 1771)
Stock material or miscellaneous articles
Longitudinally sectional layer of three or more sections
Next to unitary sheet of equal or greater extent
C428S316600, C428S309900, C428S317900, C428S086000, C428S074000, C428S318600, C052S782100, C052S790100, C052S794100
Reexamination Certificate
active
06740381
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to sandwich panel composite structures comprising fiber reinforced low density closed cell material, fibrous skin reinforcements and resin, and in particular to improved structural configurations, improved methods of resin infusion and methods of production.
BACKGROUND OF THE INVENTION
Structural sandwich panels having cores comprised of low density closed cell material, such as plastic closed cell foam, and opposing skins comprised of fibrous reinforcing mats or fabrics in a matrix of cured resin have been used for many decades in the construction of a wide variety of products, for example, boat hulls and refrigerated trailers. The foam core serves to separate and stabilize the structural skins, resist shear and compressive loads, and provide thermal insulation.
The structural performance of sandwich panels having foam cores may be markedly enhanced by providing a structure of fibrous reinforcing members within the foam core to both strengthen the core and improve attachment of the core to the panel skins, for example, as disclosed in Applicant's U.S. Pat. No. 5,834,082. When porous and fibrous reinforcements are introduced into the closed cell foam core and a porous and fibrous skin reinforcing fabric or mat is applied to each face of the core, adhesive resin, such as polyester, vinyl ester or epoxy, may be flowed throughout all of the porous skin and core reinforcements by differential pressure, for example under a vacuum bag. While impregnating the fibrous reinforcements, resin does not saturate the plastic foam core because of its closed cell composition. The resin then co-cures throughout the reinforced structure to provide a strong monolithic panel.
It is desirable to produce sandwich panels of enhanced structural performance by improving the structural connections and support among reinforcing members within the foam core and between the core and the panel skins. This is desirable in order to resist buckling loads in the reinforcing members, to prevent premature detachment of reinforcing members from one another and from the skins under load, and to provide multiple load paths for the distribution of forces applied to the panel.
Existing fiber reinforced core products offer important improvements over unreinforced foam in this regard but fail to integrate fully the separate reinforcing elements of the core into a unified and internally supported structure. For example, in a grid-like configuration of fibrous reinforcing sheet-type webs in which a first set of continuous webs is intersected by a second set of interrupted or discontinuous webs, the webs do support each other against buckling. Thus, under severe loading conditions, the discontinuous webs tend to fail at the adhesive resin bond to the continuous webs along their narrow line of intersection. This tendency may be substantially reduced by providing resin filled fillet grooves in the foam along the lines of intersection as disclosed in the above mentioned patent. However, since the reinforcing fibers of interrupted webs terminate at each intersection with a continuous web, the structural contribution of those fibers is substantially less than that of the fibers of the continuous webs.
In the case of strut or rod type core reinforcements comprising rovings of fiberglass or carbon fiber or other fibers which extend between the faces of the core, individual struts within a given row of struts may intersect each other in a lattice configuration. This supplies buckling support to each strut, but only in the plane of the strut row. To achieve bidirectional support, struts of a first row must extend through the filaments of struts of an intersecting row. This requires difficult and costly levels of accuracy and control in machine processing, since all struts must be precisely positioned in three dimensions.
SUMMARY OF THE INVENTION
One embodiment of the present invention overcomes the limitations of both web type and strut type reinforced foam cores by combining these two types of reinforcing elements into hybrid reinforcement configurations. In hybrid architecture the foam core is provided with parallel spaced rows of fibrous reinforcing webs or sheets which extend between the faces of the foam board at an acute or right angle. A second set of parallel spaced rows of reinforcing elements comprising rod-like fibrous rovings or struts also extend between the faces of the foam board at acute or right angles, and the rovings or struts intersect the webs and extend through them. Thus webs and struts constitute an interlocking three dimensional support structure in which all reinforcing fibers within the core are uninterrupted. The interconnected webs and struts provide multiple load paths to distribute normal loads efficiently among the reinforcing elements of the core and between the core structure and the panel skins. Impact damage tends to be limited to the immediate area of impact, since the complex reinforcement structure resists the development of shear planes within the core.
In an alternate hybrid architecture, the webs comprise a continuous sheet of fabric or mat which is formed into corrugations having segments which extend between the faces of the core, and the voids between the corrugations are filled with foam strips of matching cross-section. The corrugations, together with the intersecting panel skins, may form, in cross-section, rectangles, triangles, parallelograms or other geometric shapes which are structurally efficient or which offer manufacturing advantages.
In a particularly cost efficient version of hybrid core, the core reinforcing webs are produced by winding relatively low cost fibrous rovings in a helical manner onto rectangular foam strips, rather than by adhering substantially more expensive woven or stitched fabric to the surface of the foam strips. Additional rovings may be applied axially along the length of the strips during the winding operation to enhance structural properties of the strips or to serve as low cost components of the finished panel skins. The fiber-wound foam strips may also be attached together to form a structural core without the addition of rows of structural struts. In this configuration, the contiguous or adjacent sides of wound strips of rectangular cross section form web elements having I-beam flanges for attachment to panel skins. In contrast to the disclosure of U.S. Pat. No. 4,411,939, the fibrous extensions of each core web are attached to panel skins on both sides of the web rather than only one, greatly increasing the shear strength of the resulting panel. This permits the use of lighter and less expensive webs for a given strength requirement. Similarly, the present invention provides markedly improved core-to-skin attachment and shear strength when compared to the structure disclosed in Applicant's U.S. Pat. Nos. 5,462,623, 5,589,243 and 5,834,082. In tests, webs comprised of circumferentially wound rovings exhibit 75% greater shear strength than those whose end portions terminate adjacent the panel skins. Each wound strip may be provided with internal transverse reinforcing webs to provide bi-directional strength and stiffness. Roving-wound cores may also be formed using strips of triangular cross section.
The winding of rovings by machine and the consolidation of the fiber-wound strips into a single core have both economic and handling advantages. It is common for a single composite bridge deck panel or yacht hull constructed in accordance with U. S. Pat. No. 5,701,234, 5,904,972 or 5,958,325 to comprise a thousand or more individual core blocks. The labor component of producing these individual cores is very high. Reinforcement fabric is cut into sheets which are wrapped and glued around each separate core, or smaller pieces of fabric are glued to the separate faces of each core, or tubular fabrics are first formed and the cores inserted into them. These processes become increasingly difficult as the dimensions of the core components decrease. Arrangement of these cores in a mold is also labor in
Campbell G. Scott
Day Stephen W.
Hutcheson Daniel M.
Jacox Meckstroth & Jenkins
Morris Terrel
Vo Hai
Webcore Technologies, Inc.
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